The present invention relates to an audio signal encoder, and more particularly to an audio signal encoder that includes: a mapping transform unit for subjecting input audio signals to mapping transform to generate frequency region signals that vary in response to frequency variation (also referred to as frequency domain signals, which are expressed as a function defined with respect to a frequency domain); a code amount designation unit that supplies, as a code amount, a coding bit rate set or designated by a user; and a frequency region signal compression encoder that, based on the code amount designated by the code amount designation unit, subjects frequency region signals to compression encoding processing to generate a bitstream.
One example of an audio signal encoder of the prior art is described in Digital Audio Compression Standard AC-3 issued by the Advanced Television System Committee (referred to hereinbelow as Reference 1). FIG. 1 is a block diagram of the audio signal encoder described in Reference 1. The audio signal encoder of the prior art shown in FIG. 1 is provided with: bandwidth-limiting filter 20, mapping transform unit 11, code amount designation unit 12, and frequency region signal compression encoder 13.
Bandwidth-limiting filter 20 eliminates a frequency component that is not the object intended to encode from the input audio signals. Mapping transform unit 11 executes a mapping transform process on the input bandwidth-limited audio signals to generate frequency region signals. Code amount designation unit 12 transfers a coding bit rate that has been designated by the user to frequency region signal compression encoder 13. Based on the coding bit rate supplied by code amount designation unit 12, frequency region signal compression encoder 13 executes compression-coding processing on the frequency region signals to generate a bitstream.
In the above-described audio signal encoder of the prior art, the frequency components, which are included in the input audio signals but are not intended to encode, are removed through bandwidth-limiting filter processing in bandwidth-limiting filter 20. As an example, the use of a 3-Hz high-pass filter is recommended in the section on Input Filtering in Chapter 8.2. 1.3 of the above-described Reference 1.
However, this bandwidth-limiting filtering typically requires a large number of product-sum operations, and thus has the problem of entailing a large amount of operations.
The bandwidth-limited audio signals are subject to a mapping transform in mapping transform unit 11 and converted to frequency region signals. In Reference 1, a Modified Discrete Cosine Transform (MDCT) is used as the mapping transform to generate MDCT coefficients. The MDCT coefficients are frequency region signals that specify the behavior of the input audio signals through the use of frequency as a variable. The Modified Discrete Cosine Transform is widely used as a mapping transform means in audio encoding, and since the details regarding such aspects as calculation formulas of this means are widely known from documents such as Reference 1, explanation is here omitted. In Reference 1, a single Modified Discrete Cosine Transform normally generates 256 MDCT coefficients.
The MDCT coefficient represents spectrum intensity of an input audio signal with respect to frequency.
Code amount designation unit 12 supplies a coding bit rate that has been predetermined or that has been designated by a user to frequency region signal compression encoder 13.
Frequency region signal compression encoder 13 subjects the MDCT coefficients that have been generated by mapping transform unit 11 to information compression so as to meet the coding bit rate designated by code amount designation unit 12 and generates a bitstream. The information compression in this case includes entropy coding of quantized values, suppression of signal redundancy among a plurality of channels, and quantization based on auditory characteristics that are generally widely used in audio encoding. These techniques are generally widely known from documents such as Reference 1, and because these techniques have no relation to the novelty of the present invention, explanation regarding the details of these techniques is here omitted.
As previously described, the problem of the audio signal encoder of the above-described prior art is a large number of product-sum operations required for the filter processing of the bandwidth-limiting filter to result in a large amount of operations of the bandwidth-limiting filter.
It is an object of the present invention to eliminate the signals of a frequency zone which are not the object of coding by means of a small amount of operations and thereby improve the performance of an audio signal encoder, and further, to increase the speed of the encoding process, reduce power consumption, improve integration, and finally, simplify the circuits and the device construction.